1. Field of the Invention
The present invention relates generally to signal input selecting circuits, and more particularly, is directed to an improvement in a circuit having a plurality of signal input terminals and operative to select one of a plurality of input signals supplied respectively from a plurality of audio signal producing apparatus which are connected with the signal input terminals, respectively.
2. Description of the Prior Art
In the field of audio apparatus for performing sound reproduction, there has been proposed a sound reproducing system in which a plurality of audio signals generated respectively by a plurality of audio signal producing apparatus including, for example, a tape recorder operating with a pair of independent supply and take-up tape reels, a tape recorder operating with a tape cassette, an audio disc player and so on, are applied respectively to a plurality of external input terminals of an audio amplifier and subjected selectively to amplification by the audio amplifier, and an amplified audio signal obtained from the audio amplifier is applied to a speaker for generating reproduced sound. The audio amplifier which is one of main structural components of the sound reproducing system is disposed so as to have the external input terminals thereof connected to output terminals of the audio signal producing apparatus, respectively, and in operation is operative to select one of the audio signals applied to external input terminals and to amplify the selected audio signals. Therefore, a signal input selecting circuit including the external input terminals is provided in the audio amplifier. A part of a sound reproducing system provided with a typical signal input selecting circuit proposed previously is constituted as shown in FIG. 1. In the part of the sound reproducing system shown in FIG. 1, an audio amplifier 11 having three signal input terminals 12, 13 and 14 is provided. The signal input terminal 12 comprises a signal electrode 12S and a reference electrode 12E, the signal input terminal 13 comprises a signal electrode 13S and a reference electrode 13E, and the signal input terminal 14 comprises a signal electrode 14S and a reference electrode 14E.
The reference electrodes 12E, 13E and 14E constituting the signal input terminal 12, 13, and 14, respectively, are coupled in common with a chassis 15 which is provided with a reference potential in the audio amplifier 11, and the signal electrodes 12S, 13S and 14S constituting the signal input terminals 12, 13 and 14, respectively, are coupled with fixed contacts 20a, 20b and 20c of a switch 20, respectively. A movable contact 20d of the switch 20 is selectively connected to the fixed contacts 20b and 20c and a signal obtained through the movable contact 20d of the switch 20 is supplied to a signal processing block 21 contained in the audio amplifier 11.
In the audio signal reproducing system having such a part as mentioned above, the signal input terminals 12 to 14 and the switch 20 constitute the signal input selecting circuit proposed previously. The chassis 15 is equivalently grounded through a stray capacitor C1.
The signal input terminals 12 to 14 are connected to an output terminal 26 of an audio signal producing apparatus 25, an output terminal 29 of an audio signal producing apparatus 28 and an output terminal 32 of an audio signal producing apparatus 31, respectively. The output terminal 26 of the audio signal producing apparatus 25 comprises a signal electrode 26S and a reference electrode 26E. The signal electrode 26S is coupled with a signal supplying block 27 contained in the audio signal producing apparatus 25 and the reference electrode 26E is coupled with a chassis 34 contained in the audio signal producing apparatus 25. Further, the signal electrode 26S and the reference electrode 26E are connected, through a connecting cable 35, to the signal electrode 12S and the reference electrode 12E of the signal input terminal 12 provided in the audio amplifier 11, respectively.
The output terminal 29 of the audio signal producing apparatus 28 comprises a signal electrode 29S and a reference electrode 29E. The signal electrode 29S and the reference electrode 29E are coupled with a signal supplying block 30 and a chassis 38 contained in the audio signal producing apparatus 28, respectively, and further connected through a connecting cable 36 to the signal electrode 13S and the reference electrode 13E of the signal input terminal 13 provided in the audio amplifier 11, respectively.
Similarly, the output terminal 32 of the audio signal producing apparatus 31 comprises a signal electrode 32S and a reference electrode 32E. The signal electrode 32S and the reference electrode 32E are coupled with a signal supplying block 33 and a chassis 39 contained in the audio signal producing apparatus 31, respectively, and further connected through a connecting cable 37, to the signal electrode 14S and the reference electrode 14E of the signal input terminal 14 provided in the audio amplifier 11, respectively.
With the configuration thus constituted, when the movable contact 20d is connected to the fixed contact 20a in the switch 20, a first operation mode wherein the signal input terminal 12 of the audio amplifier 11 is selected and thereby a signal from the signal supplying block 27 contained in the audio signal producing apparatus 25 is supplied through the output terminal 26 of the audio signal producing apparatus 25, the signal input terminal 12 and the switch 20 in the audio amplifier 11 to the signal processing block 21 contained in the audio amplifier 11 is provided; when the movable contact 20d is connected to the fixed contact 20b in switch 20, a second operation mode wherein the signal input terminal 13 of the audio amplifier 11 is selected and thereby a signal from the signal supplying block 30 contained in the audio signal producing apparatus 28 is supplied through the output terminal 29 of the audio signal producing apparatus 28, the signal input terminal 13 and the switch 20 in the audio amplifier 11 to the signal processing block 21 contained in the audio amplifier 11 is provided; and when the movable contact 20d is connected to the fixed contact 20c in switch 20, a third operation mode wherein the signal input terminal 14 of the audio amplifier 11 is selected and thereby a signal from the signal supplying block 33 contained in the audio signal producing apparatus 31 is supplied through the output terminal 32 of the audio signal producing apparatus 31, the signal input terminal 14 and the switch 20 in the audio amplifier 11 to the signal processing block 21 contained in the audio amplifier 11 is provided.
Under the signal input selecting circuit constituted as described above with the signal input terminals 12 to 14 and the switch 20 is employed in the audio amplifier 11, potential differences between the chassis 15 in the audio amplifier 11 and each of the chassis 34 in the audio signal producing apparatus 25, the chassis 38 in the audio signal producing apparatus 28 and the chassis 39 in the audio signal producing apparatus 31 provide noise sources between the audio amplifier 11 and each of the audio signal producing apparatus 25, 28 and 31. For example, with regard to a combination of the audio amplifier 11 and the audio signal producing apparatus 25, a noise source Sn is equivalently connected through a stray capacitor C2 to the chassis 34 in the audio signal producing apparatus 25 and a noise current from the noise source Sn flows through a reference conductor of the connecting cable 35 which connects the reference electrode 12E of the signal input terminal 12 in the audio amplifier 11 and the reference electrode 26E of the output terminal 26 of the audio signal producing apparatus 25 with each other, so that a noise voltage corresponding to the product of the noise current and impedance Z1 rendered by the reference conductor of the connecting cable 35 arises. Similarly, with regard to a combination of the audio amplifier 11 and the audio signal producing apparatus 28, a noise current flows through a reference conductor of the connecting cable 36 which connects the reference electrode 13E of the signal input terminal 13 in the audio amplifier 11 and the reference electrode 29E of the output terminal 29 of the audio signal producing apparatus 28 with each other, so that a noise voltage corresponding to the product of the noise current and impedance Z2 rendered by the reference conductor of the connecting cable 36 arises. Further, with regard to a combination of the audio amplifier 11 and the audio signal producing apparatus 31, a noise current flows through a reference conductor of the connecting cable 37 which connects the reference electrode 14E of the signal input terminal 14 in the audio amplifier 11 and the reference electrode 32E of the output terminal 32 of the audio signal producing apparatus 31 with each other, so that a noise voltage corresponding to the product of the noise current and impedance Z3 rendered by the reference conductor of the connecting cable 37 arises.
Since the reference electrodes 12E, 13E and 14E of the signal input terminals 12 to 14 are coupled in common with the chassis 15 in the audio amplifier 11, each of the noise voltages arising between the audio amplifier 11 and the audio signal producing apparatus 25, between the audio amplifier 11 and the audio signal producing apparatus 28 and between the audio amplifier 11 and the audio signal producing apparatus 31, respectively, is added, through one of the signal input terminals 12 to 14 selected by the switch 20, to the signal supplied from the switch 20 to the signal processing block 21 in the audio amplifier 11 and therefore the signal supplied to the signal processing block 21 is deteriorated in a signal to noise ratio (S/N ratio).